1. Field of the Invention
The invention relates to a process for detecting channels which is used in a reception device carrying out a frequency scan of a transmission band.
2. Description of the Related Art
Television reception devices currently employ automatic channel searching in order to simplify their operational setup. Thus, a user can configure his television or his satellite decoder by pressing just one button.
Automatic searching consists in frequency scanning the entire reception band used by the apparatus and then in storing all the carrier frequencies as well as certain information relating to the channel received. For a satellite decoder, the reception band to be scanned may for example be 1 GHz and makes such scanning relatively lengthy.
To carry out the scanning of the band, it is known to effect a scan based on frequency spacing. The frequency spacing is fixed to be less than the width of a channel so that if a channel is present somewhere in the reception band it will inevitably be come across. During a frequency incrementation, the signal corresponding to the frequency is measured and compared with a threshold. If the signal is greater than the threshold, the frequency is made to vary positively and negatively to determine the carrier frequency of the channel. The channel is then decoded to obtain and to store the information relating to the channel found.
Satellite-based transmissions use various channel widths. By way of example, one and the same satellite can broadcast channels of width 25, 33 or 40 MHz. Scanning is thus performed with a spacing corresponding to the smallest channel width. To mark the channel type, it is necessary to check during each frequency jump which channel has been found with the aid of a successive identification on each channel type.